You find ISSN number of each issue of the journal. If you use only single online article, then you should look for the address of the publisher, you should go to his website - or you use world wide catalogues where all publications are visible, also journals, also about your university library available. - Note: Not the single article but the journal has an ISSN number, it is independent of the published year.
The ISSN number is an international journal identification number. Each published journal is identified by this number.
This number could be found on the journal, or on the journal homepage, or in library record where the journal is stored, or in the Ulrich's globe guide to periodical literature.